For a variety of panel members, such as outer panels and inner panels, and cover members, such as heat insulators and floor undercovers, of a vehicle, a resin-foam core composite plate formed by sandwiching a core of a resin foam between metal plates such as steel plates or aluminum plates is used (for example, see Patent Document 1). This is aimed at giving weight reduction, damping performance, and sound insulating performance of the vehicle.
The panel members and the cover members of the vehicle are frequently fastened to other members of the same part or a counterpart member such as a frame member by resistance welding or bolt fastening. However, since the soft resin foam that does not conduct electricity is interposed as the core, these members formed of the resin-foam core composite plate cannot be fastened by the resistance welding. Further, there is a drawback in that these members cannot be robustly fastened to the counterpart member even by the bolt fastening.
As a means for robustly fastening the resin-foam core composite plate to the counterpart member by the bolt fastening, a fastening structure configured to insert bolts into bolt holes formed in the composite plate and to interpose a cylindrical member, which is made up of a body part longer than a thickness of the composite plate and flange parts, between the bolt and the bolt hole is proposed. In this structure, by tightening the body part of the cylindrical member using the bolt, front and rear metal plates are sandwiched between the body part, which is deformed to protrude toward a core layer side, and the flange part or the counterpart member. Further, similarly, there is a proposal for a fastening structure configured to interpose a plurality of disc springs having a conical surface between a bolt hole and a bolt inserted into the bolt hole (for example, see Patent Document 2). In this structure, by tightening the disc springs using the bolt, front and rear metal plates are sandwiched between the disc springs, which are flatly deformed to protrude toward a core layer side, and a washer or the counterpart member.